The present invention relates to machining of materials, and more particularly to the precision machining of materials using lasers. The present invention improves the accuracy of laser machining of metal alloys, ceramics, polymers, and other materials, provides an economical method for treatment of metals to improve corrosion resistance, and allows better control of defects such as particulates in pulsed laser deposition (PLD).
Materials processing using lasers has grown greatly in past decade. Materials processing consists of material removal steps such as drilling, cutting, as well as joining. The majority of lasers now used are CO2 or Nd:YAG lasers with continuous wave (CW) or long-pulsed (ms long pulses at 100s of Hz rates) temporal formats. The long-pulsed formats typically consists of millisecond pulse lengths at a frequency in the hundreds of Hz.
Laser machining consists of focusing laser light to ablate materials for the purpose of cutting a hole, slice, etc. Conventional laser machining methods typically exhibit several limitations, including the fact that a high laser fluence, typically greater than 10 joules/cm2, is required in order to initiate the ablation. Conventional lasers typically cannot achieve these high levels of fluence. Additionally, conventional lasers typically adversely affect a larger portion of the target material than desired as a result of heat flowing out of the ablated zone and into the surrounding material. Additionally, the effectiveness of conventional laser machining is very dependent on the composition of the target material and the wavelength of light, which are typically not optimal for minimizing negative effects on the surrounding material.
Basic physics constraints on the stored energy per unit volume and gain sets the output of single-rod Nd:YAG lasers at about 1 kW, CO2 lasers at about 6 kW, and excimer lasers at a few hundred watts, which is inadequate for many machining processes. Since these conventional lasers can""t be scaled to the output power ( greater than 10 kW) required in many machining applications, their use hasn""t become prevalent. When a conventional laser is used to machine difficult to ablate materials, such as metals or ceramics, the high power levels required tend to create negative side effects, such as a large beat affected zone, cracks, or melted areas surrounding the machined area.
Applications of high average power lasers with outputs above 10 kW mainly fall into two categories: thermal processing or ablative processing. Thermal processing includes surface texturing, surface amorphization, laser glazing and annealing, adhesive bond pretreatment, crystallizing amorphous silicon, laser annealing, deposition, and cutting for photovoltaics, and solvent-free cleaning. Ablative processing includes micromachining, cutting and slitting, and deposition of large area thin films.
Prior art methods of micromachining metals include the use of continuous wave lasers. The CW lasers must heat the target material enough to create some surface transformation. In micromachining with CW lasers, such as creating holes in an aluminum surface, using an existing art CO2 laser at a 6 kW average power level requires a great deal of time, perhaps 4 to 5 milliseconds, to achieve the desired surface transformation. An example may be the micromachining of the leading edge of an airplane wing to improve boundary layer characteristics. In this example, billions of tiny holes are machined on the leading edge of a wing. The machining time per hole with prior art laser systems is unacceptably large. Applications such as this require a much faster and more efficient micromachining method to do the job in a reasonable amount of time.
Machining applications therefore remain unexploited because they are best performed by a laser with an ultrashort pulse and high pulse repetition frequency (PRF) output with high average power. Ultrashort laser pulses are typically defined as short optical pulses in the femtosecond (10xe2x88x9215 s) to picosecond (10xe2x88x9212 s) range. What is needed is a laser system employing ultrashort pulses in the femtosecond (fs) to picosecond (ps) range at a PRF of 100 kHz to MHz range and at average power levels above 10 kW.
Applications of high power (above 10 kW) lasers fall into one of two categories: thermal processing or ablative processing. Conventional ultrafast (short-pulsed) lasers have not yet achieved high average power, and based on the nature of the amplification process, aren""t likely to. Compelling reasons to use ultrafast lasers include: 1) a lower threshold for ablation, 2) more deterministic damage zone, 3) ablation with minimal creation of a heat-affected zone, 4) no cracking or melting, and 5) reduced time to create the desired machining effect. Along with a short-pulse time structure, other desirable properties of a high power laser are a high PRF (approximately 100 kHz to 300 MHz) and wavelength agility, so absorption bands (if present) in the material can be accessed. Basic physics constraints on the energy density achievable with lasers based on non-FEL methods means buying more lasers in order to increase the power available for a material processing procedure. Over and beyond space constraints, the complexity of then combining the output of these lasers and possibly synchronizing them as well comes into play.
U.S. Pat. No. 5,656,186 by Mourou, et al., issued Aug. 12, 1997 (hereinafter the ""186 patent), discloses a method for laser-induced breakdown of a material with a pulsed laser beam in which the material is characterized by a relationship of fluence breakdown threshold versus laser beam pulse width that exhibits at least a clearly detectable and distinct change in slope at a predetermined laser pulse width value. The ""186 patent proposes generating a beam of laser pulses focused to a point at or beneath the surface of the target material and wherein each pulse has a pulse width equal to or less than the predetermined laser pulse width value. In employing the method of the ""186 patent to ablate silver film on glass, a Ti:Sapphire laser generated pulse widths of 7 ns to 100 fs at 800 nm wavelength, a frequency of 1 kHz, a spot size of 3.0 xcexcm diameter, and a fluence of 0.4 joules/cm2. The method of the ""186 patent is, however, limited in its usefulness for machining materials such as metals and ceramics. Although the Ti:Sapphire laser of the ""186 patent can deliver short pulses, it does not permit the high average power levels that are required to adequately machine these materials. This has limited the adoption of conventional lasers for machining these types of materials.
As shown by the above discussion, there is a need for laser machining of materials that exhibits a lower threshold for ablation, more deterministic damage, ablation with minimal heat-affected zone in metals, no cracking or melting, and reduced time to create the desired machining effect, the desired surface treatment, or the desired pulsed laser deposition of material.
It is therefore an object of the present invention to provide a laser machining system in which 1) ablation is stimulated at a lower power threshold, 2) the damage zone is more controllable, 3) the heat affected zone is minimal, 4) cracking and melting is avoided, and 5) the time is greatly reduced.
These, and other advantages of the machining method of the present invention will become readily apparent to one of skill in the art after reading the attached description and with reference to the attached drawings and appended claims.
The present invention combines the benefits of an ultrashort pulse laser at high average power with a continuous wave laser. An initial pulse would be provided by a laser with an ultrashort pulse duration of between 100 and 600 femtoseconds at a fluence of 0.5 J/cm2 or greater focused on the target. Simultaneous to or shortly after this pulse, a pulse from a continuous wave laser with a pulse length of between 100 nanoseconds and 1 microsecond and at a fluence of 1 J/cm2 would also be applied to the target. The second laser pulse would sustain and enlarge on the ablation effect launched in the initial pulse. The pulse pairs are repeated as often as desired to produce the desired micromachining effect. The ultrashort pulses may be supplied by a titanium sapphire laser at 1 millijoule and 1.0 micron wavelength or by an FEL at 100 microjoules and 0.3 micron wavelength. The continuous wave laser may be a CO2 laser at 1 joule and 10.6 microns wavelength or by a Nd:YAG laser at 1 joule and 1.06 micron wavelength. Many other laser configurations are possible to achieve the desired criteria of an initial ultrashort pulse to ignite the ablation followed by a longer pulse to sustain the machining action.